The invention relates to the production of circuits by microlithography and more specifically relates to a method for the optical alignment of masks and patterns located in close or near planes and the alignment apparatus used for performing this method.
In devices for reproducing designs on a pattern by exposure to X-radiation or light radiation through an appropriate mask located at a few micrometers from the pattern each operation requires a precise alignment of mask and pattern. To obtain this alignment with an accuracy of approximately 1/10th of a micrometer a number of alignment methods using the diffraction properties of light by gratings have been described, for example the Moire fringes method or the methods of Torii and Smith. These methods comprise inscribing on the mask and on the pattern gratings having a predetermined spacing and detecting the radiation diffracted by these two gratings located in near planes.
The Moire fringes method comprises inscribing on the mask and pattern two gratings with slightly different spacings. The combination of the different order of diffraction gives a fringes system whose spread increases in inverse proportion to the spacings of the gratings. The improvement in accuracy sought by this spatial amplification of the fringes is lost in coherent light by noise problems (speckle, interference, etc. ) and in incoherent light by the poor contrast of the fringes.
The Torii method comprises inscribing complimentary gratings on the mask and on the pattern, each of the gratings having absorbent zones of width l and transparent zones of width 3.times.l. Illumination is by monochromatic light. The two gratings are intercalated when the alignment is performed. In the aligned position such a method makes it possible to detect one extremum in a given direction, but the position of this extremum is dependent on the spacing between the planes to be aligned.
Finally the Smith method comprises inscribing on both mask and pattern two gratings of the same spacing, illuminating by a monochromatic plane wave and measuring the intensity diffracted by the two gratings in the resulting symmetrical orders of diffraction (e.g. the resultant order +1 and the resultant order -1). The alignment of the two gratings is obtained when the two signals are equal. The disadvantage of this method is the high sensitivity with respect to spacing of the two planes and with respect to the symmetry of the gratings obtained.
All these methods leave an uncertainty due to the periodicity of the resultant signal and it is therefore necessary to remove this uncertainty by another method.